This should be a minimal C-API needed to deal with Set objects. It
supports creating the sets, checking whether an element is the set,
adding and removing elements, iterating over the elements, clearing
a set, and returning the size of the set.

Co-authored-by: Nobuyoshi Nakada <nobu.nakada@gmail.com>

This header file is simply out of date (for decades since at least
1989).  It's the 21st century.  Just stop using it.

To fix build failures.

This shall fix compile errors.

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

Split ruby.h

This was related to $SAFE, and was deprecated in 2.7.  I missed it
earlier when removing the other $SAFE-related code.

Would like to edit them in forthcoming commit.

Inside the block where `RB_METHOD_DEFINITION_DECL` family are
defined.